A machine tool such as a lathe performs cutting with a work fixed to a chuck of a main spindle and a cutting tool abutting against the rotating work. An inaccurate contact between the work and a seating face of the chuck in this case leads to extra cutting of the work and a reduction in finish quality. PTL 1 and PTL 2 disclose work seating detection devices that detect an appropriate seating of the work with respect to the main spindle chuck. In these work seating detection devices, compressed air is supplied from an air supply source to a detection hole that is formed in the chuck of the main spindle and a back pressure of the compressed air is detected by a pressure switch. In a state where the work is appropriately seated, the compressed air rarely leaks, and thus a high back pressure is obtained. In a case where the work is in a state of inappropriate seating, a large amount of the compressed air leaks from a gap and the back pressure is reduced. A confirmation of whether or not the work is appropriately seated with respect to the main spindle chuck is performed by the back pressure being detected.